The invention described herein relates to a technique and structure for producing very high resolution color images using a monochrome charge-coupled device (CCD) imager and a rapid sequential color illumination scheme. More specifically, the present invention provides a full-frame monochrome CCD architecture which produces an aggregate color image signal separable into component color image signals.
Conventional methods for obtaining color electronic snapshots have traditionally employed one of three techniques. The first of these techniques uses unfiltered illumination of a color CCD imager (FIG. 1). A color CCD imager 10 comprises a CCD imager upon which color filter stripes 14, 16, and 18 are deposited on sensor elements resulting in an array of color pixels 12. The color pixels are generally sensitive to the negative primary colors, cyan, yellow, and magenta.
The second technique uses unfiltered (white light) illumination of a color CCD imaging module. As illustrated by FIG. 2, a color CCD imaging module 20 employs three monochrome CCD imagers 22, 24, and 26 placed behind a beam-splitter 28. The beam-splitter separates the incoming radiation 30 from the imaging optics into three beams of different spectral bands (typically, red 32, green 34, and blue 36), with each CCD imager sensing and storing one of the monochrome images. Both of these techniques can be used to obtain snapshots of subject matter with brief exposure time (e.g., for moving objects), however the cost of both is relatively high due to such factors as, the manufacturing cost of the color CCD, or the cost of the beam-splitter, the special optics, and the three CCDs.
A third technique, as shown in FIG. 3, employs a system 40 which uses a monochrome CCD imager with a sequential color illumination scheme. A filter wheel 42 is rotated in front of the imager 44 or the light source 46 so that the imager is exposed to a series of the color images which are then read out sequentially. This technique is less expensive than the previously described techniques, but has a limitation in the nature of the subject matter for which it can be used because the total exposure time is relatively long.
This problem of relatively long exposure time is aggravated in applications which employ a full-frame CCD imager architecture. In a very high resolution CCD image sensor it has not been practical to incorporate separate storage areas. As a result of the fact that there is no separate storage area or areas, image integration and readout times have to be successive rather than simultaneous as with other architectures used, for example, in video applications. Because of the need to shield the imager during successive image read-outs the total exposure time must include three integration times (one for each color) and two read-out times.
FIG. 4 shows a timing diagram which is representative of a typical exposure time. The top line 47 represents the three individual exposure times; the high state being when the CCD imager is exposed to an image. The middle line 48 similarly represents the three individual read-out times. The bottom line 49 represents the total exposure time.
Beginning at t.sub.1, the imager is exposed to the blue portion of the image (see integration time line 47). The image charge integrated during the exposure is then read out beginning at time t.sub.2 (read-out time line 48). After the blue portion of the image is downloaded, the imager is exposed to the red portion of the image beginning at t.sub.3. The red portion of the image is then downloaded beginning at time t.sub.4. Finally, beginning at t.sub.5 the imager is exposed to the green portion of the image which is read out beginning at t.sub.6. Thus, as illustrated on the total exposure time line 49, this imaging scheme requires three exposures and two read-out intervals before a complete color image can be obtained. In addition, the high density of pixels and sequential nature of the downloading of imaging information further increases the total exposure time, sometimes taking as long as five to ten seconds for a complete color image to be stored. The result being that this technique is unsuitable for creating images of moving subject matter even if the individual exposure times are very short.
FIG. 5 shows a simplified representation of a conventional three-phase full-frame CCD imager 50 which can be used with any of the above-described techniques. Three gate electrodes 52 that control each individual storage region 54 (one such region shown outlined in bold lines) are driven respectively by one set of clock signals .phi..sub.1, .phi..sub.2, and .phi..sub.3. Charge which is stored in each row of storage regions is shifted sequentially into a horizontal CCD register 56 from which it is read out serially through output 58.
One approach to rapid sequential exposure is provided by U.S. Pat. No. 4,989,075 to Ito for SOLID STATE IMAGE SENSOR DEVICE (Ito). In Ito, a solid state image sensing device is described which employs an array of photoelectric conversion members , which has three separate sets of vertical transfer arrays associated therewith. Each set of vertical transfer arrays is coupled to an associated horizontal transfer array. The array of photoelectric conversion members is illuminated with three successive color images, one set of vertical transfer arrays being selected to store charge for each exposure. Thus, the charge generated from the three successive exposures is stored in the three different sets of vertical transfer arrays, one set of vertical transfer arrays for each color. The charge stored in each set of vertical transfer arrays is then read out one horizontal row at a time via the associated horizontal transfer array. Thus, a rapid sequential exposure may be accomplished using one image sensing array. However, the requirement that each vertical column of photoelectric conversion members in the array have three corresponding opaque vertical transfer arrays makes the device of Ito not only architecturally complex, but excessively large for certain applications. Roughly 75% of the device is devoted to opaque storage area rather than imaging area, resulting in an unacceptably low fill factor for very high resolution applications.
In view of the preceding discussion, it is apparent that there exists a need for a full-frame, color CCD imager which combines the low cost of sequential color illumination with the speed of the more costly white light illumination schemes.